Levees Holding Back Isaac in New Orleans

By By Alex Sosnowski, expert senior meteorologist.

September 01, 2012, 3:10:08 AM EDT

According to the United States Army Corps of Engineers, the levees are holding in the New Orleans, despite ongoing battering by Isaac for an extended period.

In a statement released on the Army Corps website, "The New Orleans Hurricane and Storm Damage Risk Reductions System (HSDRRS) is performing as designed and we are confident in the system."

The system includes a series of pumps, levees, canals and gates.

Earthen levees in Plaquemines Parish were being overtopped early Wednesday, but they are outside of the federally designed and maintained system.

The Corps was providing assistance in Plaquemines Parish and will continue to monitor the HSDRRS throughout the Isaac event, according to the site.

Storm Surge

Storm surge levels are peaking generally as forecast by AccuWeather.com. However, levels are remaining high a bit longer than forecast, due to the stall that occurred with Isaac early Wednesday morning.

Forward motion of Isaac will resume and the storm will weaken now that the center is drifting over land. Since Isaac is a large system, above-normal tide levels will continue throughout the day Wednesday and can still occur to some extent into Thursday.

A significant storm surge occurred in the Shell Beach, La., area with levels between 9 and 11 feet above normal during the late evening hours Tuesday and the early morning hours Wednesday.

The greatest storm surge, perhaps near 12 feet based on twitter photographs, has occurred in part of Plaquemines Parish, near the mouth of the Mississippi River. The storm surge was enhanced in this area due to Isaac strengthening upon making landfall and the angle at which the water was being directed toward the coast.

Tidal backup was forecast to produce major flooding of the Jourdan River in southernmost Mississippi later Wednesday by National Weather Service hydrologists. Prior low water levels on the Mississippi River prevented flooding around New Orleans.

At Pascagoula, Miss., a storm surge of about 4 feet was occurring and peaking during the midday hours Wednesday.

Levels of Lake Pontchartrain at New Orleans were running about 6 feet above normal, well within what the levee system should handle.

In Grand Isle, La., a storm surge of around 5 feet was occurring Wednesday midday. Above-normal levels will continue in the area into Wednesday night.

Farther west in the Morgan City, La., area, the circulation around Isaac was producing below-normal tide levels. However, even as the storm diminishes, its northwest movement and eventual southern winds will produce above-normal tides later Wednesday into Wednesday night.

Farther east, in Mobile Bay, Ala., a storm surge of 3 to 5 feet above normal will continue through Wednesday. Tides will gradually return to normal by later Thursday.

In the Panama City, Fla., area, tide levels will continue to run between 1 and 3 feet above normal, due to the southerly flow off the Gulf of Mexico from the big storm.